Japan's largest bank apologized Monday for the alleged theft by an employee of more than 1 billion yen ($6.6 million) from customers' safe deposit boxes.

The bank, formally known as Mitsubishi UFJ Financial Group, Inc. (MUFG), said Monday that it was investigating and that verified thefts from about 20 of the 60 clients thought to have been affected amounted to 300 million yen (nearly $2 million). Compensation was being worked out, it said.

The thefts occurred at two Tokyo branches of the bank from April 2020 until the end of October this year, when the problem was discovered.

MUFG’s president and CEO Junichi Hanzawa told reporters the female employee responsible for managing the safe deposit boxes and their keys is suspected of taking advantage of that position to steal cash and other valuables.

"The case has undermined the customers' trust and confidence and shaken the very foundation of our banking business," Hanzawa said. He apologized for causing concern and trouble to MUFG clients.

MUFG said the employee admitted taking the money and using it for investments and her personal purposes. She was fired and was cooperating with the bank's internal probe and a police investigation but has not yet been arrested.

After news of the thefts surfaced, dozens more people came forward with claims of suspected losses that the bank is working to verify, it said.

The MUFG case is the second recent one involving criminal activity at a major Japanese financial institution. In November, Nomura Holdings acknowledged that a former employee had been arrested on suspicion of robbery, attempted murder and arson targeting a client in Hiroshima.

Asked about the MUFG thefts, the chief government spokesperson said Monday that banking operations require high levels of trust and a sense of public service.

The case was "extremely regrettable," said Chief Cabinet Secretary Yoshimasa Hayashi.